DESCRIPTION

These functions make queries to and interpret the responses
from Internet domain name servers.

The res_init() function reads the configuration files
(see resolv+(8)) to get the default domain name, search
order and name server address(es). If no server is given,
the local host is tried. If no domain is given, that
associated with the local host is used. It can be overridden
with the environment variable LOCALDOMAIN. res_init()
is normally executed by the first call to one of the other
functions.

The res_query() function queries the name server for
the fully-qualified domain name name of specified
type and class. The reply is left in the
buffer answer of length anslen supplied by the
caller.

The res_search() function makes a query and waits for
the response like res_query(), but in addition
implements the default and search rules controlled by
RES_DEFNAMES and RES_DNSRCH (see description of _res
options below).

The res_querydomain() function makes a query using
res_query() on the concatenation of name and
domain.

The following functions are lower-level routines used by
res_query().

The res_mkquery() function constructs a query message
in buf of length buflen for the domain name
dname. The query type op is usually QUERY, but
can be any of the types defined in
. newrr is currently
unused.

The res_send() function sends a pre-formatted query
given in msg of length msglen and returns the
answer in answer which is of length anslen. It
will call res_init(), if it has not already been
called.

The dn_comp() function compresses the domain name
exp_dn and stores it in the buffer comp_dn of
length length. The compression uses an array of
pointers dnptrs to previously compressed names in the
current message. The first pointer points to the beginning
of the message and the list ends with NULL. The limit of the
array is specified by lastdnptr. if dnptr is
NULL, domain names are not compressed. If lastdnptr
is NULL, the list of labels is not updated.

The dn_expand() function expands the compressed domain name
comp_dn to a full domain name, which is placed in the
buffer exp_dn of size length. The compressed
name is contained in a query or reply message, and
msg points to the beginning of the
message.

The resolver routines use global configuration and state
information contained in the structure _res, which is
defined in . The only field that is
normally manipulated by the user is _res.options.
This field can contain the bitwise ``or'' of the following
options:

RES_INIT

True if res_init() has been called.

RES_DEBUG

Print debugging messages.

RES_AAONLY

Accept authoritative answers only. res_send()
continues until it fins an authoritative answer or returns
an error. [Not currently implemented?.